In present-day mechanical engineering, it is routine for steel to be used at temperatures lower than minus 100° C., for example in compressors. Most grades of steel have a ferritic/martensitic crystal structure and become very brittle at these low temperatures. Accordingly, these grades of steel cannot be used for many applications at minus 100° C. This could be remedied by using other steels, namely tough-at-subzero steels, instead of grades of steel with a ferritic/martensitic crystal structure. Tough-at-subzero steels are characterized by their austenitic structure and are comparatively soft. In addition, these tough-at-subzero steels have low strength.
In the case of solid bodies, toughness is to be understood as meaning the property of being able to undergo macroscopically measurable plastic deformation under mechanical stress. Toughness may also refer to the degree of resistance with which a body opposes a plastic change of shape, i.e. the level of mechanical stress and/or energy that has to be exerted to produce deformation. Brittleness may be considered to be the converse property.